The recent increase in the use of portable electronic devices such as mobile telephones and notebook computers has created a need for smaller, lighter, longer lasting rechargeable batteries to provide the power to the above mentioned and other battery powered devices. During the 1990s, lithium rechargeable batteries, specifically lithium-ion batteries, became popular and, in terms of units sold, now dominate the portable electronics marketplace. However, as more and more power hungry functions are added to the above mentioned devices (e.g. cameras on mobile phones), improved batteries that store more energy per unit mass and per unit volume are required.
It is well known that silicon can be used as the active anode material of a rechargeable lithium-ion electrochemical battery cell (see, for example, Insertion Electrode Materials for Rechargeable Lithium Batteries, M. Winter, J. O. Besenhard, M. E. Spahr, and P. Novuk in Adv. Mater. 1998, 10, No. 10). The basic composition of a conventional lithium-ion rechargeable battery cell is shown in FIG. 1 including a graphite-based anode electrode, the component to be replaced by the silicon-based anode. The battery cell includes a single cell but may also include more than one cell.
The battery cell generally comprises a copper current collector for the anode 10 and an aluminium current collector for the cathode 12 which are externally connectable to a load or to a recharging source as appropriate. A graphite-based composite anode layer 14 overlays the current collector 10 and a lithium containing metal oxide-based composite cathode layer 16 overlays the current collector 12. A porous plastic spacer or separator 20 is provided between the graphite-based composite anode layer 14 and the lithium containing metal oxide-based composite cathode layer 16 and a liquid electrolyte material is dispersed within porous plastic spacer or separator 20, the composite anode layer 14 and the composite cathode layer 16. In some cases, the porous plastic spacer or separator 20 may be replaced by a polymer electrolyte material and in such cases the polymer electrolyte material is present within both the composite anode layer 14 and the composite cathode layer 16.
When the battery cell is fully charged, lithium has been transported from the lithium containing metal oxide via the electrolyte into the graphite-based layer where it reacts with the graphite to create the compound, LiC6. The graphite, being the electrochemically active material in the composite anode layer, has a maximum capacity of 372 mAh/g. It will be noted that the terms “anode” and “cathode” are used in the sense that the battery is placed across a load.
It is generally believed that silicon, when used as an active anode material in a lithium-ion rechargeable cell, provides a significantly higher capacity than the currently used graphite. Silicon, when converted to the compound Li21Si5 by reaction with lithium in an electrochemical cell, has a maximum capacity of 4,200 mAh/g, considerably higher than the maximum capacity for graphite. Thus, if graphite can be replaced by silicon in a lithium rechargeable battery the desired increase in stored energy per unit mass and per unit volume can be achieved.
Existing approaches of using a silicon or silicon-based active anode material in a lithium-ion electrochemical cell have failed to show sustained capacity over the required number of charge/discharge cycles and are thus not commercially viable.
One approach disclosed in the art uses silicon in the form of a powder (say as particles or spherical elements with a 10 μm diameter), in some instances made into a composite with or without an electronic additive and containing an appropriate binder such as polyvinylidene difluoride coated onto a copper current collector. However, this electrode system fails to show sustained capacity when subjected to repeated charge/discharge cycles. It is believed that this capacity loss is due to partial mechanical isolation of the silicon powder mass arising from the volumetric expansion/contraction associated with lithium insertion/extraction to and from the host silicon. In turn this gives rise to electrical isolation of the silicon elements from both the copper current collector and themselves. In addition, the volumetric expansion/contraction causes the spherical elements to be broken up causing a loss of electrical contact within the spherical element itself.
Another approach known in the art designed to deal with the problem of the large volume changes during successive cycles is to make the size of the silicon elements that make up the silicon powder very small, that is to use spherical particles that have diameters in the 1-10 nm range. This strategy assumes that the nano-sized elements can undergo the large volumetric expansion/contraction associated with lithium insertion/extraction without being broken up or destroyed. However, this approach is problematic in that it requires the handling of very fine, nano-sized powder that may pose a health and safety risk and it does not prevent the electrical isolation of the spherical elements from both the copper current collector and themselves as the silicon powder undergoes the volumetric expansion/contraction associated with lithium insertion/extraction. Importantly, since a lithium-containing surface film is typically created during lithium insertion and the lithium ions that make up this surface film are trapped and can not be removed during the deinstertion process, the large surface area of the nano-sized elements can give introduce large irreversible capacity into the lithium-ion battery cell. In addition, the large number of small silicon particles creates a large number of particle-to-particle contacts for a given mass of silicon and these each have a contact resistance and may thus cause the electrical resistance of the silicon mass to be too high. The above problems have thus prevented silicon particles from becoming a commercially viable replacement for graphite in lithium rechargeable batteries and specifically lithium-ion batteries.
In another approach described by Ohara et al. in Journal of Power Sources 136 (2004) 303-306 silicon is evaporated onto a nickel foil current collector as a thin film and this structure is then used to form the anode of a lithium-ion cell. However, although this approach gives good capacity retention, this is only the case for very thin films (say ˜50 nm) and thus these electrode structures do not give usable amounts of capacity per unit area. Increasing the film thickness (say >250 nm) causes the good capacity retention to be eliminated. The good capacity retention of these thin films is considered by the present inventors to be due to the ability of the thin film to absorb the volumetric expansion/contraction associated with lithium insertion/extraction from the host silicon without the film being broken up or destroyed. Also, the thin film has a much lower surface area than the equivalent mass of nano-sized particles and thus the amount of irreversible capacity due to the formation of a lithium-containing surface film is reduced. The above problems have thus prevented a thin film of silicon on a metal foil current collector from becoming a commercially viable replacement for graphite in lithium rechargeable batteries and specifically lithium-ion batteries.
In another approach described in U.S. Pat. No. 6,887,511, silicon is evaporated onto a roughened copper substrate to create medium-thickness films of up to 10 μm. During the initial lithium ion insertion process, the silicon film breaks up to form pillars of silicon. These pillars can then reversibly react with lithium ions and good capacity retention is achieved. However, the process does not function well with thicker films and the creation of the medium-thickness film is an expensive process, thus limiting this concept's commercially viability. Also, the pillared structure created by the break up of the film has no inherent porosity and thus the long terms capacity retention is questionable.
In another approach described in US2004/0126659, silicon is evaporated onto nickel fibres which are then used to form the anode of a lithium battery. However this is found to provide an uneven distribution of silicon on the nickel fibres hence significantly affecting operation. In addition, these structures have a high ratio of nickel current collector mass to active silicon mass and thus do not give usable amounts of capacity per unit area or per unit mass.
A review of nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells has been provided by Kasavajjula et al (J. Power Sources (2006), doi:10.1016/jpowsour.2006.09.84), herewith incorporated by reference herein.